A waste-minimized protocol for copper-catalyzed Ullmann-type reaction in a biomass derived furfuryl alcohol/water azeotrope

Article abstract of DOI:10.1039/c8gc00287h

We report the use of biomass-derived furfuryl alcohol as an effective bidentate ligand able to promote the Ullmann-type copper-catalyzed coupling of aryl halides with heteroaromatic or aliphatic amines. Furfuryl alcohol (FA) can be mixed with water to form the corresponding azeotrope (20 wt% of FA) and therefore can be easily recovered and reused. This protocol is efficiently applied to substrates with various electronic nature and affords the expected products (27 examples) in generally good to excellent yields. It has also been demonstrated that the protocol is both chemically and environmentally effective as the azeotropic mixture can be easily and almost quantitatively recovered at the end of the process.

Full text of DOI:10.1039/c8gc00287h

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DOI: 10.1039/C8GC00287H
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ARTICLE
A Waste-Minimized Protocol for Copper-Catalyzed Ullmann-Type
Reaction in a Biomass Derived Furfuryl Alcohol/Water Azeotrope
Francesco Ferlin, Valeria Trombettoni, Lorenzo Luciani, Soliver Fusi, Oriana Piermatti, Stefano
Santoro and Luigi Vaccaro*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We report the use of biomass-derived furfuryl alcohol as an effective bidentate ligand able to promote the Ullmann-type
copper-catalyzed coupling of aryl halides with heteroaromatic or aliphatic amines. Furfuryl alcohol (FA) can be used in
mixture with water to form the corresponding azeotrope (20%wt of FA) and therefore can be easily recovered and reused.
This protocol is efficiently applicable to substrates with various electronic nature and affords the expected products (27
examples) in generally good to excellent yields. It has been also demonstrated that the protocol is both chemically and
environmentally effective as the azeotropic mixture can be easily and almost quantitatively recovered at the end of the
process.
www.rsc.org/
coupling product through a reductive elimination, which would
also regenerate the Cu(I)-catalyst.7
Despite the great variety of reported methodologies and the
above-mentioned recent advances, no specific effort has been
Introduction
N-Arylation strategies are undoubtedly among the most useful
reactions in organic chemistry. In fact, the aromatic amine
moiety is almost ubiquitous in materials that find wide
applications in different areas such as medicinal chemistry,1
agrochemistry,2 and material sciences.3A great number of
catalytic protocols for the classical Ullmann reaction have been
disclosed and copper-catalyzed N-arylations can be considered
nowadays as a valid alternative to the palladium-catalyzed
Buchwald–Hartwig protocols.4 Copper-based methods offer
advantages in terms of price and toxicity, although higher
loadings, harsher reaction conditions and high-boiling aprotic
solvents such as DMF, DMSO, toluene and butyronitrile are
often required.4 Progress towards highly chemically efficient
and greener conditions has however recently been made, with
protocols appearing in the literature using homeopathic copper
loadings,5 but also with examples of copper-catalyzed N-
arylations in aqueous media,6 generally employing phase
transfer catalysts.
reported for the definition of a protocol for the Ullmann-type
coupling combining chemical efficiency and waste-minimization
green chemistry restrictions. One of the most critical aspects
that need to be addressed at this aim is the selection of the
reaction medium, which is well known to be responsible for
most of the waste generated in organic syntheses, particularly
in pharmaceutical industries.8 In this perspective, the shift from
fossil-derived to biomass-derived9 and most importantly
recoverable solvents, would represent
a
significant
improvement in terms of sustainability, particularly for large
scale productions.
Following this line of thought, our research group has recently
demonstrated that an accurate choice of solvent and reaction
conditions can allow the access to chemically effective
conditions and also the drastic reduction of waste, as proven by
the E-factor calculation.10-12 We have also proposed the use of
γ-valerolactone (GVL), a chemical deriving from lignocellulosic
biomass,11 as an efficient and greener alternative to common
polar aprotic solvents in several palladium-catalyzed reactions,
including cross-couplings and C–H functionalizations.12 Looking
for other biomass-derived chemicals that could find potential
application as reaction media in chemical processes, we
identified furfuryl alcohol (FA) as a possible candidate. This
chemical is produced from hemicellulose, and finds its main
application in the synthesis of resins and as a precursor for the
synthesis of other chemicals.13
Although still not completely understood, the most widely
accepted mechanism for this type of transformation involves a
Cu(I)/Cu(III) catalytic cycle. This would involve the initial
formation of a copper(I)-nucleophile species, promoted by the
use of an inorganic base such as K3PO4 or K2CO3, that would then
undergo an oxidative addition with the aryl halide. The
generated copper(III)-intermediate would then afford the
FA is soluble in common organic solvents (e.g. ethyl acetate,
chloroform, acetone), and is miscible with water. Very
interestingly, although FA has a relatively high boiling point (171
°C), it forms an azeotrope with water (20 wt% of FA) that boils
at 98.5 °C, a convenient temperature for distillation. Besides,
Laboratory of Green Synthetic Organic Chemistry – Dipartimento di Chimica,
Biologia e Biotecnologie, Università di Perugia Via Elce di Sotto, 8 - 06123
Perugia, Italy. E-mail: luigi.vaccaro@unipg.it
† Electronic Supplementary Informaꢀon (ESI) available: experimental procedures,
calculations of E-factors and characterization data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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homogeneous mixtures of an organic solvent and water are (2 equivalents) in isopropanol, achieving higher conversion after
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often used as reaction media in several reactions, mainly to 18 h compared to the reaction making uDseOIo: f101.1,203-e9/tCh8aGnCe0d0io28l 7aHs
ensure dissolution of organic reactants as well as inorganic ligand (entry 1 vs 2). To our delight, we also found that by
additives. For this reason, we are also developing sustainable replacing isopropanol with water not only does the reaction
protocols by exploiting the use of an azeotropic mixture as an proceeded to product 3a satisfactorily, but also with slightly
effective tool to simplify the recovery and reuse of the reaction higher conversion (entry 3). 1,2-Ethanediol is also more
medium and reduce waste. We proved that this strategy is effective as a ligand in water (entry 4) than in isopropanol.
indeed applicable in a range of reactions,14 and very recently we Next, to test whether our strategy could allow for a waste
also reported the use of the FA/water az. as a recoverable and minimized protocol for the Ullmann reaction, we performed the
reusable reaction medium for the copper-catalyzed azide- reaction using a FA/water ratio corresponding to the azeotropic
alkyne cycloaddition (CuAAC). This resulted in a dramatic mixture (20 wt% of FA) (entries 5 and 6). We found that the
reduction in the amount of waste generated compared to reaction proceeds slightly better, leading to complete
commonly used literature protocols.15
conversion in 24 h and allowing isolation of the pure product 3a
Aiming at developing an efficient protocol for the Ullmann-type in 87% yield (entry 6). We also ran the reaction with isopropanol
coupling of amines with aryl halides, we speculated that the use replacing FA (entry 7). The significant drop in conversion to 3a
of an azeotropic mixture as reaction medium could be beneficial (23%) observed suggests that FA could have a more specific role
also in this case. The use of an azeotrope can in fact allow as a copper ligand. Finally, we performed the model reaction in
complete dissolution of the organic reactants as well as the pure isopropanol and in water (entries 8 and 9, respectively).
required copper catalyst and inorganic base, and at the same The reaction in water resulted in moderate conversion (54%),
time grants a simple recovery of the reaction medium. while the reaction in isopropanol only led to the formation of
Furthermore, inspired by previous studies,16 we hypothesized 7% of the coupling product 3a, perhaps because of the low
that FA could act as a bidentate ligand for copper, possibly solubility of the inorganic base. Among representative inorganic
resulting in an improved catalytic efficiency. In fact, most of the bases, potassium phosphate resulted in the best results in the
reported methodologies for Ullmann-type reactions make use FA/water az. (see Supporting Information, SI). It is also worthy
of some sort of ligand, often N- or O-based, which result in to notice that while most of the protocols using water as
higher product yields and faster reaction rates.4,6,16 Specifically, medium, propose the use of a phase transfer catalyst, in our
for the use of FA, we have been inspired by the use of the closely case FA behaves as a ligand for copper but also lead to a
related 1,2-ethanediol,16h or related moieties. Our reaction homogeneous reaction mixture.
design based on use of the FA/water az., would thus allow the
complete recovery of the reaction medium together with the
ligand at the end of the reaction by simple distillation (Scheme
Table 1. Effect of Solvents and Additives in the Ullmann-Type Coupling of 1 with 2a.a
1).
Typical Ullmann-type C–N coupling:
R1
[Cu]
base
R
R
I
N
R2
HNR1R2
+
Entry
Medium (1M)
iPrOH
Additive (2 eq.)
1,2-Ethanediol
Furfuryl alcohol
Furfuryl alcohol
1,2-Ethanediol
Conversion%b
ligand
solvent
1
2
3
4
5
6
7
8
9
42
89
92
91
95
100c
23
7
- Generally require bidentate ligands
iPrOH
H2O
H2O
- Often catalyst, ligand and solvent are not recoverable
Waste-minimized Ullmann-type coupling:
CuI
R1
N
FA/H2O az.(20 wt% of FA)
FA/H2O az.(20 wt% of FA)
H2O
iPrOH
H2O
R
I
R
R2
K3PO4
HNR1R2
+
iPrOH
-
-
Furfuryl alcohol/water
azeotrope
54
- Renewable biobased furfuryl alcohol as bidentate ligand
- Ligand/solvent system easily recoverable by distillation
- Dramatic reduction in waste-generation
aReaction conditions: Imidazole (1 eq.), iodobenzene (1 eq.), CuI (1 mol%), K3PO4
c
(2 eq.), additive (2 eq.), 150 °C, 18 h. bDetermined by GC analysis. 24 h reaction
time with 87% isolated yield of the pure product.
Scheme 1. Features of current work.
Intrigued by these results, we investigated the role of FA as
bidentate ligand by performing the Ullmann reaction of 1 using
Results and discussion
related 1,2-diols/ether ligands (Scheme 2). Generally, all the
tested bidentate diols and also their methyl ether gave
comparable and very good results. This confirms the
effectiveness of these easily accessible bidentate ligands and
the beneficial effect of water as medium for the Ullmann-type
To prove our strategy, we started the investigation by
performing the model reaction between imidazole and
iodobenzene 2a in different combinations of solvents and
1
additives (Table 1). First, we used furfuryl alcohol as an additive
reaction of imidazole 1. FA and tetrahydro-FA showed a slightly
2 | J. Name., 2012, 00, 1-3
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better efficiency allowing the complete disappearance of
starting materials and the exclusive formation of the desired
product 3a. FA is however more functional in our strategy as it
is the only one of these dioxygenated ligands able to form an
azeotropic mixture with water, also in the optimal conditions
for promoting the Ullmann-type process.
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DOI: 10.1039/C8GC00287H
Scheme 2. Role of FA related ligands in the Ullmann-type coupling of 1.
Scheme 3. Substrate scope for imidazole arylation. Reaction conditions: imidazole (1
eq.), aryl iodide (1 eq.), CuI (1 mol%), K3PO4 (2 eq.), FA/H2O azeotrope [1M], T = 150 °C,
a
b
c
24 h. All yields are isolated yields. Reaction time 21 h. Reaction time 40 h. T = 90 °C.
dReaction time 12 h.
We then tested the influence of FA amount in the aqueous
medium (see SI), finding that optimal results are obtained with
a substrate/FA ratio of 1:2 (Table 1, entries 4-6). Finally, it is
worth-considering that this ratio also corresponds to the use of
1M FA/water az. (20 wt% of FA in water), which represents our
final target to access a protocol combining high chemical
efficiency and minimal waste production.
To ultimately confirm the utility of the optimal reaction
conditions and the essential advantage offered by the use of the
FA/water azeotrope over previously reported protocols, we
verified the possibility to recover both reaction medium and
additive/ligand. On a representative 30 mmol scale, after
Next, we proceeded the investigation on the substrate scope by
considering less explored and more challenging nitrogen-
containing heterocycles, such as indole and carbazole (Scheme
4). To account for the reduced reactivity of these substrates
compared to imidazole, the catalyst loading was increased to 5
mol%. In our reaction conditions, the N-arylation of indole
occurred efficiently with isolated yields for the desired products
5a-h ranging from 73% to 88%. Pleasingly, also in this case the
reaction was insensitive to the electronic nature of the aryl
iodide, and was tolerant to the presence of keto-groups (5d) or
chloride substitutions (5h). As already observed for the
arylation of imidazole, the presence of a nitro-group required
lower temperatures and longer reaction time (5f). Carbazole
also proved to be a competent substrate in our conditions,
although leading to product 5i-n in lower yields (40-65%) and
longer reaction times (48-72 hours). Besides the expected
reactivity limit, it is noteworthy that chloride functionalities on
the aryl iodide remained untouched during the reaction.
completion of the reaction between imidazole
(1) and
iodobenzene (2a), distillation of the reaction medium allowed
the recovery of 93% of the FA/water az., whose purity and
composition was confirmed by 1H NMR analysis.
Therefore, with our optimal reaction conditions based on the
use of FA as a ligand in water, we extended the protocol to the
reactions of imidazole (1) with a series of aryl iodides 2 using,
otherwise, rather standard conditions, namely 1 mol % of CuI as
catalyst and K3PO4 as base (Scheme 3). All the reactions
proceeded efficiently at 150 °C in 12-24 h regardless of the
electronic nature of the substituents, providing the coupling
Finally, we also tested our reaction conditions in few examples
of N-arylation of primary alkyl amines with either simple
iodobenzene or 4-iodoanisole (Table 2). The desired products
were isolated in yields ranging from 55% to 80%.
7
products
3 in generally good yields. The presence of a ketone
functionality on the aryl iodide coupling partner was well
tolerated (3e). Moreover, iodoarenes possessing a chloride
substitution reacted selectively at the iodine position (3g and
3h), thus allowing, in principle, for consecutive orthogonal
To complete the study, we also quantified the advantage
obtainable in terms of waste produced, by calculating the E-
factor (Kg of waste/Kg of product) associated with our protocol.
Considering the recyclability of the recovered azeotrope (ca.
93%), we calculated a value as low as 9.7. It should be
considered that for related processes most of the procedures
reported in the literature are associated with much larger waste
production, with E-factors usually in the order of hundreds (see
SI for details). The recovered furfuryl alcohol/water azeotrope
proved to pure and completely clean from any traces of
products; it was reused expectedly giving identical results.
functionalizations. The reaction of 1-iodo-3-nitrobenzene (2f
)
was not efficient in our standard reaction conditions, due to
partial decomposition of the starting material. However, by
reducing the temperature to 90 °C (but in longer reaction time),
product 3f was obtained with 60% yield.
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medium and activating system for the Ullmann-VtyiepweArtcicolepOpnelinre-
catalyzed coupling of aryl halides wiDthOI:h1e0.t1e0r3o9a/Cro8GmCa0t0ic287oHr
aliphatic amines. The protocol is efficiently applicable to
substrates with various electronic nature and affords the
expected products in generally good yields. Importantly, our
results suggest that furfuryl alcohol does not merely act as a co-
solvent, but instead facilitates the reaction in a more specific
fashion, arguably acting as a ligand for copper. We also
demonstrated that our protocol is both chemically and
environmentally effective as the azeotropic mixture can be
easily and almost quantitatively recovered at the end of the
process, which contributes to a dramatic reduction in waste-
generation compared to established literature procedures.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The Università degli Studi di Perugia is acknowledged for
financial support under the program “Ricerca di Base 2015”. We
gratefully acknowledge the “Fondazione Cassa di Risparmio di
Perugia” for financial support through the project
“Manipolazione di biomassa lignocellulosica per la produzione
di additivi per biocarburanti”, Codice Progetto: 2016.0060.021
RICERCA SCIENTIFICA E TECNOLOGICA. This work was also
supported by the IREU program of the National Science
Foundation under award number DMR-1262908 via the
American Chemical Society. The Università degli Studi di Perugia
and MIUR are also acknowledged for financial support to the
project AMIS, through the program “Dipartimenti di Eccellenza
- 2018-2022”.
Scheme 4. Substrate scope for indole and carbazole arylation. Reaction conditions:
indole or carbazole (1 eq.), aryl iodide (1 eq.), CuI (5mol%), K3PO4 (2 eq.), FA/H2O
azeotrope [1M], T = 150 °C, 30 h. All yields are isolated yields. aReaction time 48 h. bT =
90 °C. cReaction time 12 h. dReaction time 72 h.
Notes and references
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Entry
R1
R2
H
H
OMe
OMe
Yield %b
75c (7a)
55 (7b)
80 (7c)
75 (7d)
2
1
2
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nHex
nOct
nHex
nOct
3
4
G. D’Aprano, M. Leclerc, G. Zotti and G. Schiavon, Chem.
Mater., 1995, 7, 33.
aReaction conditions: imidazole (1 eq.), aryl iodide (1 eq.), CuI (5 mol%), K3PO4 (2
eq.), FA/H2O azeotrope [1M], T = 150 °C, 48 h. bAll yields are isolated yields.
cReaction conducted with 2 eq. of 2.
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In conclusion, we demonstrated that the furfuryl alcohol/water
azeotrope can be efficiently used as a recoverable reaction
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